I'm pretty sure the Kit Car series and the Locost series require full cages these days. Sprinting and hillclimbing call for MSA spec roll bar and rear stays and also sometimes a forward facing brace tube that tends to run down into the passenger footwell.
Bent tubes will always want to bend that's why the powers that be don't like them on upright legs of a roll bar. Saying a tube can be bent to follow the door pillar etc is fine if fitted in a saloon car like a Mini etc but a 7 doesn't have doors..
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If I was to race a 7 I would want a full cage with side impact bars as I have seen what happens when it goes wrong. And putting Kit Cars which are 99% clad in grp in amongst steel bodied cars is dangerous in my opinion.
Cages made from T45 are lighter by the way,the spec of the material allows thinner walled tube to be used.....it's added goodness in my book of goodness adding .